1. Field of the Invention
This invention relates to Bragg grating reflectors, and more particularly narrow bandwidth Bragg grating reflectors for use in optical waveguides.
2. Description of the Related Art
The formation of Bragg reflection gratings in photosensitive optical fibers is described in Hill et al., "Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask," Applied Physics Letters, Vol. 62, No. 10, Mar. 8, 1993, pp. 1035-1037, and G. Meltz et al., "Formation of Bragg gratings in optical fibers by a transverse holographic method", Optics Letters, vol. 14, no. 15, August 1989, pages 823-825. Such gratings have been used to fabricate optical fiber lasers, such as the one described in G. A. Ball et al., "Design of a Single-Mode Linear-Cavity Erbium Fiber Laser Utilizing Bragg Reflectors," Journal of Lightwave Technology, Vol. 10, No. 10, October, 1992, pp. 1338-1343.
A Bragg grating will generally reflect light that falls within its frequency band, whose width (bandwidth) is inversely related to the length of the grating. Therefore, for devices that require Bragg reflectors with a very narrow frequency band, such as single-mode fiber lasers, the Bragg grating must be made relatively long.
There are currently two methods of fabricating Bragg gratings in optical fibers: (1) Two-beam interference method and (2) Phase mask method. In the two-beam interference method, described in the Meltz article cited above, a transverse process is used in which the Bragg grating is written in the core of the photosensitive fiber by exposing it to a two-beam interference pattern. The two interfering beams create light and dark interference fringes in the fiber core, which cause a corresponding variation in its refractive index. The length of the resulting Bragg grating is determined by the fiber core area that the two interfering beams illuminate, which in turn is limited by the diameter of the beams.
To write uniform gratings using the two-beam interference method, the two writing beams must consist of perfect plane waves (they must be perfectly collimated) with uniform intensities over the overlapping beam areas that are used to create the interference pattern in the fiber core. Any variations in the wavefront shape of the two beams will result in a "chirp" in the interference pattern, and a corresponding chirp in the Bragg grating. Variations in beam intensity also result in fiber grating chirp, because the varying UV exposure creates a background index change. As the diameters of the writing beams are increased, it becomes increasingly difficult to maintain uniform intensities and wavefronts.
In the phase mask method, described in Dana Z. Anderson et al., "Phase-Mask Method for Volume Manufacturing of Fiber Phase Gratings", Proceedings of the Optical Fiber Conference, February 1993, paper PD16-1, pages 68-70, a single optical beam is passed through a phase mask (a phase grating), which is usually designed to diffract the beam into only two of the many possible diffraction orders.. The fiber is positioned in close proximity to (but not in direct contact with) the phase mask. The diffracted orders, which have the same function as the writing beams in the two-beam interference method described above, interfere in the fiber core and produce an index grating with a period that is equal to the phase mask grating period.
With this method, the length of the resulting fiber grating is dependent on the diameter of the single optical beam that is passed through the phase mask. As discussed above, the spatial uniformity of an optical beam becomes increasingly difficult to control as its diameter increases. The uniformity of the fiber Bragg grating is also dependent on how uniform the phase mask's periodicity is over the area that is illuminated by the optical beam. Currently available phase masks exhibit acceptable uniformity over only 2-3 centimeters.
The above limitations place a limit on the length of the Bragg gratings that can be formed with either method (generally gratings can be made no longer than 2-3 centimeters), and therefore a limit on how narrow the grating's bandwidth can be made.